The present invention relates to a data processing apparatus, a tamper detection apparatus, a data processing method, a tamper detection method, a data processing program and a tamper detection program. Particularly, this invention relates to an apparatus, a method and a program for accurately determining whether digital video data has been tampered with even after it had been encrypted but before subjected to coding processing including orthogonal transform.
Distribution of digital contents, such as video and music, has been popular not only through package media but also communications networks, such as the Internet.
Several techniques have been proposed for determining whether those digital contents have been tampered with while they are distributed.
One popular digital authentication technique for detecting tampering uses a hash function which is disclosed, for example, in Japanese Unexamined patent Publication Nos. 11 (1999)-196392, 2000-122861 and 2002-016596.
A hash function is an operational function to disturb and compress long data into a hash value of fixed length such as 128 bits. It is a one-way function h_K by which it is almost impossible to find “x” that establishes y=h_K(x) when “y” is given.
In use of the hash function for temper detection, “x” is data (having any length), “y” is a hash value (having a fixed length) and h_K is changeable by a private key “K” known by only persons who transfer the data “x” and receive it.
In this digital authentication, data “X” is transferred with a hash value. Both data “X” and a hash value may be encrypted for higher security.
A person who receives the data “x” calculates a hash value and determines that the data “x” has not been tampered with if the calculated value is equal to the transferred hash value “y” whereas it has been tampered with if the former value is not equal to the latter.
Popular hash functions are MD 5 (Message Digest 5) and SHA (Secure Hash Algorism) which are explained in detail in a no-patent literature “Cryptography for Beginners” written by Eiji Okamoto and published by Kyoritu Publishing Co.
Video data usually contain a huge amount of data, so that they are mostly subjected to irreversible compression with orthogonal transform, such as JPEG (Joint Photographic Experts Group) or MPEG (Moving picture Experts Group) before distribution.
Irreversible compression achieving high compression rate is often used in distribution of video data through a communications network for shortening a transfer time.
The digital authentication technique using a hash function determines that data has been tampered with even if one digit of the data has been modified.
Thus, this digital authentication technique requires information for finding tempering to be added to video data after data compression. In other words, addition of such information before compression of the video data causes change in a lot of bits, which makes finding tempering impossible. Moreover, once such information is added, video data cannot be reprocessed.
Therefore, the known digital authentication technique causes many problems to video-data distributors in data handling and distribution.